Bandpass or bandstop microwave filters constituted by capacitor-coupled rejector-circuit resonators are well-known. However, fabrication of such filters for use in the microwave frequency field is attended by difficulties since, at these frequencies, tuning capacitors for the resonant circuits and resonator-coupling capacitors have very low values of capacitance which cannot readily be measured or adjusted.
For example, the construction of filters operating at frequencies of a few hundred MHz leads to capacitor values of a few tenths of a picofarad in the case of tuning capacitors and of a few picofarads in the case of coupling capacitors. It is accordingly apparent that these conditions are not favorable for mass production of these types of filter since their electrical characteristics are not readily reproducible from one filter to another.
One solution to these difficulties is often provided by resonators in which quarter-wave lines short-circuited at their ends are formed by means of microwave-circuit fabrication techniques. Another solution lies in the so-called "helix" filters which are also constituted by quarter-wave lines short-circuited at the ends and wound on a mandrel, the inter-resonator couplings of these two types of filter being obtained by mutual induction between resonators.
The disadvantage of these filters, however, is that they result in bulky structures and are consequently ill-suited to certain particular applications requiring small-size and low-weight structures as is required, for example, in all components used in the manufacture of telecommunications satellites.